This invention relates generally to methods and techniques for the analysis of vibration data in oil and gas well drilling. More particularly, this invention relates to the use of frequency modulation sidebands observed from detected frequency domain vibration data for (1) discriminating between surface and downhole vibratory sources and (2) the determination of the rotary speed of bottom-hole-assembly (BHA) components.
During drilling, various sources excite the drillstring. The amplitude of the resultant drillstring vibrations will depend on the level (severity) of the excitation, the system damping and the proximity of the excitation frequency to a natural frequency of the drillstring. When the frequency of any of the excitation sources is a natural frequency of the drillstring (axial, torsional or lateral) then the string resonates. Vibration levels are generally highest at resonance, but high level vibrations may exist in the drillstring, independent of drillstring resonance, whenever a high level of excitation is present.
Torsional vibration often manifests itself as a stick/slip action of the bottomhole assembly (BHA). Indicators of these downhole vibrations can be monitored at or near the surface. This stick/slip phenomenon is described in a paper entitled "A Study of Slip-Stick Motion of the Bit" by A. Kyllingstad and G. W. Halsey, Society of Petroleum Engineers (SPE) Paper 16659, Sep. 1987. As discussed in that paper, torsional oscillations are caused by alternating slipping and sticking of the bottom hole assembly (BHA) as it rotates in the borehole. This phenomenon is associated with a large amplitude, sinusoidial and often saw-tooth like variation in the applied torque. The term slip-stick motion refers to the belief that the amplitude of the torsional oscillations becomes so large that the drillcollar section periodically comes to a complete stop and does not come free until enough torque is built up in the drillstring to overcome the static friction.
Drilling with large amplitude vibrations will result in accelerated drillstring fatigue. A recent study of drillstring failures indicated that fatigue was the primary cause of the examined failures (see Hill, T. H.; Seshadri, P. V.; Durham, K. S., "A Unified Approach to Drillstring Failure Prevention," SPE/IAOC 22002, Mar. 1991, Amsterdam). Most current efforts aimed at understanding and controlling drillstring vibrations focus on the failure of drillstring components. However, during drilling, the drillstring transfers power from the surface to the bit and high amplitude drillstring vibrations may represent a loss, or waste, of drilling energy. Therefore, high levels of vibration not only result in drillstring component failures but can also result in sub-optimum drill rates.
The avoidance of high vibration levels can be attempted in two ways: (a) the BHA can be modeled and a harmonic analysis performed to predict the operating conditions, weight-on-bit (WOB) and rotary speed (RPM), which avoid resonant conditions or (b) the vibrations can be directly monitored while drilling to determine the optimum operating conditions (WOB, RPM and pump rate).
The use of modelling analysis is limited by the number of unknowns that occur in real-time drilling operation. In addition, most models focus on drillstring resonance frequencies, and estimate bit rotary speeds which will avoid exicting these frequencies ("critical speed" analysis). Therefore, they do not take into account high level excitations independent of this resonance analysis.
There is a continuing need for improved methods of using drillstring vibration data to enhance or optimize drilling performance and/or drillstring performance. For example, there is a need for a method to locate the source (e.g., surface or downhole source) of drillstring vibration so that corrective action may be taken to prevent drillstring fatigue and less than optimum drilling rates. In addition, there is a continuing need for methods of using drillstring vibration data to determine the rotary speed of BHA components to enhance or optimize the life of the BHA components and the drilling performance.